1. Field of the Invention
The present invention relates to clearance measuring devices and methods and, more specifically to a clearance measuring device and method used for adjusting a clearance between a photomask and a plate in an exposure apparatus.
2. Description of the Background Art
Recently, a method of transferring a pattern on a photomask onto a plate by exposure while ensuring a small clearance between the photomask and the plate has been widely used. Such an exposure method is called an off contact exposure method or proximity exposure method.
In the off contact exposure method and proximity exposure method, a clearance between a photomask and a plate must be measured and adjusted by a highly reliable method. For example, the clearance between the photomask and the plate is generally adjusted to be from 0.05 mm to 0.5 mm.
Now, a conventional method of measuring a clearance between a photomask and a plate will be described with reference to FIG. 12. Shown in FIG. 12 are a clearance measuring device 300 for measuring the clearance between the photomask and the plate, and plate 101 and photomask 102 spaced apart by a clearance G5.
Clearance measuring device 300 includes in a box 301: a laser light source 302 for diagonally and downwardly directing light 302a to plate 101 and photomask 102; and a light sensor 303 sensing the light reflected by plate 101 and photomask 102.
Light 302a is divided into light 302b reflected by a surface 102a of photomask 102 which is opposite to plate 101, and light 302c reflected by a surface 101a of plate 101 which is opposite to photomask 102. Light 3O2b and 302c are sensed by light sensor 303, and a distance S5 between light 302b and 302c is calculated by a processor (not shown).
Here, as distance S5 is proportional to clearance G5, clearance G5 can be preliminary adjusted to provide a given distance S5.
However, conventional clearance measuring device 300 is expensive. If a plurality of clearance measuring devices 300 must be provided to measure the clearance between plate 101 and photomask 102 at a plurality of locations, the problem associated with a cost arises.
Further, conventional clearance measuring device 300 requires a processor in addition to the device used for aligning photomask 102 and plate 101. As a result, the problem associated with the cost arises and the structure of the clearance measuring device becomes complicated.
Moreover, conventional clearance measuring device 300 requires laser light source 302 and light sensor 303 in addition to these at used for aligning photomask 102 and plate 101. Thus, the structure of the clearance measuring device becomes complicated.
When the pattern on photomask 102 is transferred onto plate 101 by exposure, a mechanism for moving clearance measuring device 300 outside photomask 101 must be provided. Thus, the structure of the clearance measuring device becomes complicated.